Thin coatings of silicone polymers on substrates such as film or paper have come to form an integral component in the construction of numerous products in the pressure sensitive adhesives industry, especially in the manufacture of labels, where such coated sheets are usually referred to as release liners. The level of release, or peel force required for such applications, is in most instances very low. Silicone materials have been found to be especially well suited for this purpose due to the low tendency for pressure sensitive adhesives to adhere to surfaces coated with substantially pure polydimethylsiloxane. A number of materials have been developed which consist primarily of polydimethylsiloxane substituted with small amounts of various reactive functional groups for chemically crosslinking the liquid silicone on the substrate after coating, and which usually requires some period of heating in an oven to effect the cure before the pressure sensitive adhesive can be applied. Of course, it is essential for the performance of the final product that the silicone coating be completely cured, and that the cure be completed as quickly as possible for feasibility of manufacture.
The complete and rapid cure of silicone coatings has been a problem in the past. Recently, however, a number of reactive silicones have become available which undergo a rapid cure effected by exposure to a sufficient dose of radiation, commonly emitted from ultraviolet lights, or visible lights, electron beam (E-beam) devices or thermal devices. The selection of the radiation source depends on specific process requirements. For example, polydimethylsiloxanes substituted with small amounts of pendant acrylate groups, when combined with photoactive initiators and coated as liquid films on most substrates, upon brief exposure to radiation in an atmosphere which contains little or no oxygen crosslinks to provide solid, tack-free, premium release coatings through the free radical polymerization of the acrylic side chains, as described in U.S. Pat. No. 4,563,539 (Gornowicz et al.). Another example is that of epoxy functional silicones as described in U.S. Pat. No. 4,279,717 (Eckberg), which are commercially available from the General Electric Company. These are compositions consisting essentially of polydimethylsiloxanes in which 1-20 mole-% of the silicon atoms have been substituted with epoxyalkyl groups, preferably epoxy cyclohexyl ethyl, and small amounts of soluble catalysts, especially diaryliodonium hexafluoroantimonate. When a thin coating of this liquid formulation is applied to the substrate by a standard coating technique and then exposed to a sufficient dose of radiation, the catalyst instantly decomposes to generate traces of strong acid. This strong acid initiates the rapid cationic polymerization of the epoxy group to provide a tack-free coating, usually in a matter of seconds or less, which completely cures in a matter of minutes to hours following irradiation.
In these types of silicone release coatings, the final cured films have a structure which is essentially silicone throughout the bulk of the coating, consisting of polydimethylsiloxane segments crosslinked together into a network with small amounts of low molecular weight polycyclohexylene oxide chains. Because of this low concentration of non-silicone components, pressure-sensitive adhesives which contact these coatings release very easily, even when aged for extended periods of time at high temperatures. Heat aged peel forces from these coatings generally register in the 0.5 to 2.0 Newton per decimeter (N/dm), depending upon the particular pressure-sensitive adhesive (PSA) employed.
For many products, on the other hand, significantly higher peel forces are required than can be obtained from the pure silicone having a low content of functional groups; for example, in certain label constructions, or in release coatings for tapes in roll form (usually referred to as low adhesion backsizes). Two methods for increasing the release level of these reactive silicones have been disclosed in the patent literature. The first method involves the further substitution of silicon atoms in the polysiloxane chains with additional acrylate (U.S. Pat. No. 4,576,999, Eckberg) or epoxyalkyl groups (U.S. Pat. No. 4,313,988, Koshar), or other groups such as alkylene phenol (U.S. Pat. No. 4,952,657, Desorcie et al.). The second method is to add limited amounts of certain non-silicone co-reactive diluents to the low epoxy or acrylate functional silicones. Thus, in the case of epoxy silicones a number of suitable additives have been described, such as epoxy alkanes as described in U.S. Pat. No. 4,576,999 (Eckberg, et. al.), polyepoxide resins as in U.S. Pat. No. 4,547,431 (Eckberg), and even styrenic monomers as in U.S. Pat. No. 4,988,741 (Stein).
When small amounts of these cationically co-reactive monomers were added to the epoxy functional silicone, increases in the peel forces for removal of tape strips, roughly proportional to the amount of non-silicone component added, were observed for tape samples which were prepared by solvent casting of the pressure sensitive adhesive on the cured liner, followed by laminating a film backing to the dried psa, and then aging the tape samples. Thus, mixtures of epoxysilicone containing from 5 to about 40% by weight of reactive diluent provided cured coatings which exhibited an increasing peel force for removal of tape strips up to about 225 g/in (8.7 N/dm) at the highest loading (40%) of modifiers studied. Presumably, these added monomers efficiently co-polymerize with the silicone bonded epoxide. Based on these results and other teachings contained in the body of the prior art, it was recommended that the amounts of added reactive diluents not exceed 40% by weight of epoxysilicone, since beyond that level it was felt the release levels would climb too rapidly to be useful for pressure sensitive adhesive release purposes.
Similar findings have been described for silicone release coatings based upon other radiation-reactive functional groups in which free radical polymerization and crosslinking is used to effect the cure. U.S. Pat. No. 4,070,526 (Colquhoun et al.) discloses radiation curable compositions comprising mercaptoalkyl-substituted polydiorganosiloxane fluid, from about 1 to 50 parts by weight vinyl monomer (per 100 parts of the fluid) and, optionally, a methylvinylpolysiloxane. Upon curing, with little or no oxygen present, variable release of adhesives is said to be controllably provided. Release data for compositions containing greater than 50 parts of vinyl monomer (see Table I of U.S. Pat. No. 4,070,526) indicates that release is not reliably obtained at these higher reactive diluent levels, i.e. at lower levels of silicone. U.S. Pat. No. 4,783,490 (Eckberg et al.) discloses radiation curable compositions comprising mercapto-substituted silicon compounds, reactive co-compounds such as multifunctional acrylates, and photoinitiator. Reactive diluents, such as monofunctional acrylates, may optionally be added to control viscosity, although generally it is not desirable to add more than about 25% by weight. Co-reactive diluents, employed at levels up to about 30% by weight of the radiation-polymerizable acrylate-functional organopolysiloxane composition, are also described in U.S. Pat. No. 4,606,933 (Griswold et al.). These added diluents are desirable in aiding the adhesion of this organopolysiloxane composition to a substrate.
U.S. Pat. No. 4,201,808 (Cully et al.) discloses radiation curable release coating compositions, most commonly for paper substrates, comprising from about 10 to about 90 weight percent (based on the total weight of the composition) of "an organopolysiloxane containing an average of at least one" acryloxy and/or methacryloxy group per molecule, from about 90 to 10 weight percent of a low molecular weight free radically polymerizable acrylated polyol crosslinking agent, and from 0 to 10 weight percent of a photosensitizer. For comopsitions containing at least 50% of the (meth)acrylate functional organopolysiloxane, the polyol crosslinking agent is not siloxane based. But for compositions containing less than 50%, the crosslinking agent is siloxane based. To adjust viscosity, the compositions can also contain from 0.01 to about 30 weight percent of a reactive diluent, such as liquid organic monoacrylate ester.
Finally, U.S. Ser. No. 08/304,424, now U.S. Pat. No. 5,527,578 assigned to the assignee of the present case, describes a radiation curable release coating composition comprising a minor portion of acrylate-functional silicone (0.05 to 25% by weight of the composition), 5 to 60% by weight multifunctional free radically copolymerizable vinyl monomers, and 25 to 95% by weight monofunctional, free radically copolymerizable vinyl monomers. These release coating compositions provide stable reproducible levels of release in the intermediate region.
Despite the improvements in release coating technology that these radiation curable functional silicones have provided to the pressure sensitive adhesive products industry, these materials still suffer from a number of significant drawbacks which have precluded their use in many potential applications. The free radically polymerized materials (i.e., acrylate modified silicones) require an inert atmosphere to rapidly cure, conditions which are difficult to maintain in production environments, and also have a tendency to transfer silicone contaminants to adhesive surfaces. Cationically cured (i.e., epoxy functional silicone) release coatings, which do not require inert processing conditions and have less of a tendency to transfer silicone, are essentially pure silicone which require additional time or heat after irradiation to completely cure. However, these cured silicone films are quite fragile, due to the poor internal strength of unfilled crosslinked polydimethylsiloxane, making such coatings susceptible to abrasion and rub-off. Furthermore, although modification of release by incorporation of reactive diluents does increase the peel adhesion of the tape or label to the coating, it has only been demonstrated to be effective for solvent cast adhesives and, in practice, it has proven difficult to reproducibly achieve targeted modified release values using this technique. In addition, none of the prior art radiation curable formulations, except for the free radically curable compositions of U.S. Patent Application Ser. No. 08/304,424, are able to be applied using the elctrospray process, which is described in U.S. Pat. No. 4,748,043 (Seaver et al.) a method which is especially useful for the solventless application of thin films to irregular substrates.
Thus, a need exists for a rapidly curing silicone coating which can be rapidly and completely cured in air and which has easy release from most pressure sensitive adhesives, and with excellent internal strength and abrasion resistance.
A further need exists for a release coating which can be readily formulated to provide a full range of release levels reproducibly and reliably, even for pressure sensitive adhesives which are not solvent coated directly onto the silicone release coating.
Furthermore, a need exists for release coating compositions of sufficiently low viscosity and suitable conductivity to be applied via electrospray coating methods.
We have found such a release coating composition, release coating, and method of making said release coating.